1. Technical Field of the Invention
The present invention relates to a mesoporous composition and a method for its synthesis. The invention provides a mesoporous composition of a new category in that the templating molecule used in the synthesis comprises vitamin E. The invention further relates to the synthesis of mesoporous compositions showing controlled morphology. The invention also relates to mesoporous compositions comprising vitamin E for use in drug delivery systems, catalysis and other bioanalytical applications.
2. Description of the Prior Art
Porous substances are generally divided by pore size. For example, those having pore sizes smaller than 2 nm are classified as microporous substances, between 2 and 50 nm are classified as mesoporous substances and larger than 50 nm are classified as macroporous substances. Because of the range of their pore sizes, mesoporous materials are compatible with applications such as separation or sensing of relatively large organic molecules. Typical of the mesoporous materials are amorphous or polycrystalline solids such as pillared clays and silicates. Unfortunately, the pores in these materials are often irregularly spaced and broadly distributed in size.
There is growing interest in the use of inorganic materials as host matrices for bioactive molecules. The principal advantages of such host/guest type materials include the stability and relative inertness of the materials as well as their easy transportation as free flowing powders. Considerable synthetic effort has therefore been devoted to developing molecular sieve frameworks with pore diameters within the mesoporous range, and the development of a series of molecular sieves having a hexagonal array of uniform mesopores has been reported. A group of researchers at Mobil Oil Corporation have reported a series of mesoporous molecular sieves, named MCM-41, in U.S. Pat. Nos. 5,057,296 and 5,102,643, which are fully incorporated by reference. According to these patents, MCM-41 has a structure exhibiting hexagonal arrangement of straight channels, such as a honeycomb, on a silica plate. MCM-41 is synthesized using the cationic type surfactant, quaternary alkyltrimethylammonium salts [CnH2n+1(CH3)3N+Xxe2x88x92] and various silica sources, like sodium silicates, tetraethyl orthosilicate, or silica gel, under hydrothermal conditions. On the other hand, the mesoporous materials in the SBA series, another group of synthetic mesoporous materials, are synthesized using neutral templates.
MCM-41 synthesis has been proposed to occur through a liquid crystal templating mechanism. Researchers have proposed that the structure is defined by the organization of surfactant molecules into liquid crystals which serve as templates for the formation of the MCM-41 structure. (Beck et. al., J. Am. Chem. Soc. 114, 10834 (1992), fully incorporated by reference herein). In other words, the first step in the synthesis would correspond to the formation of a micellar rod around the surfactant micelle, which in a second step will produce a hexagonal array of rods, followed by incorporation of an inorganic array (like silica, or silica-alumina) around the rodlike structures. That is, in an aqueous solution, surfactants form a liquid crystal structure which is surrounded by silicate ions and the liquid crystal structure is associated with MCM-41 substance via a hydrothermal reaction and then, removed by calcination at a temperature of 500 to 600xc2x0 C., to prepare MCM-41.
MCM-41 has been actively researched for characterization and application by many laboratories, because their large and uniform pore sizes allow the entry of otherwise sterically hindered molecules. The pore size of MCM-41 can be adjusted in a range of from 1.6 up to 10 nm by modulating the kinds of surfactants or synthesis conditions. Additionally, the easily tailored pore size and compositional variance available, provides a versatile range of materials for applications that span from catalysis to drug delivery.
The mesoporous compositions of the present invention are synthesized using a novel templating molecule, vitamin E, which renders the mesoporous compositions of the present invention unique over those described in the art.
A composition comprising vitamin E and silica is taught in U.S. Pat. No. 5,234,695, which is fully incorporated by reference in the disclosure. The invention contemplates the addition of a flow agent to a water dispersible vitamin E composition, where the flow agent is preferably fumed silica having an average particle size of about 0.1 micron. This composition comprises vitamin E and silica and lacks any definite form i.e., represents an amorphous composition.
The composition of the present invention is unique over the amorphous composition described in U.S. Pat. No. 5,234,695, in that said composition is a mesoporous molecular sieve, displaying definite form.
It is an object of the present invention to provide novel mesoporous compositions comprising vitamin E. It is another object of the invention to provide methods for synthesizing said novel mesoporous compositions.
The mesoporous compositions are synthesized by using Vitamin E d-xcex1-tocopheryl polyethylene glycol 1000 succinate (vitamin E TPGS) as a structure-directing agent. The mesoporous compositions can be synthesized under specific conditions to produce varied morphologies including, but not limited, to gyroids, hexagons, hexagonal rods, discs, and spheres. The synthesis conditions can be modified to control particle morphology, while maintaining the hexagonal mesoporous structure. These new class of compositions have applications as drug delivery vehicles for vitamin E and other water-insoluble drugs; as a catalyst by fixation of large active complexes in the mesopores; as fiber optic sensors by depositing the mesoporous composition comprising a fluorescent dye at the tip of an optical fiber; and in the preparation of mesoporous membranes for use in separation, fuel cells, and in catalytic membrane reactors.
In accordance with an aspect of the present invention, there is provided a method for preparing a mesoporous composition, comprising the steps of:
(A) dissolving a compound possessing amphipathic properties in a solvent to form a solution;
(B) establishing the pH of the solution of step (A) between 0-3;
(C) adding a silica source to the pH-established solution of step (B) to form a mixture;
(D) aging the mixture of step (C) to form a product; and
(E) filtering, washing, and drying the product of step (D).
In accordance with another aspect of the present invention, there is provided a mesoporous composition prepared from a mixture comprising vitamin E TPGS, hydrochloric acid, and a silica source.